1. Field of Invention
The present invention pertains to the field of magnetic memories. More particularly, this invention relates to a magnetic memory cell with a reference layer having off-axis orientation.
2. Art Background
A magnetic memory such as a magnetic random access memory (MRAM) typically includes an array of magnetic memory cells. Each magnetic memory cell usually includes a data storage layer and a reference layer. Typically, the logic state of a magnetic memory cell depends on the relative orientations of magnetization in its data storage and reference layers.
The data storage layer of a magnetic memory cell is usually a layer or film of magnetic material that stores alterable magnetization states. These alterable magnetization states typically include magnetizations which form in a direction that is parallel to what is commonly referred to as the easy axis of the data storage layer. A data storage layer also usually includes magnetizations which form near its edges including edges that are perpendicular to its easy axis. The magnetizations which form near the perpendicular edges are commonly referred to as edge domains. The resulting orientation of magnetization in the data storage layer is a result of the effects of the magnetizations along the easy axis and the effects of perpendicular magnetizations in the edge domains.
The reference layer of a magnetic memory cell is usually a layer of magnetic material in which magnetization is fixed or "pinned" in a particular direction. In a typical prior magnetic memory cell, the reference layer is formed so that its magnetization is pinned in a direction which is parallel to the easy axis of the data storage layer. As a consequence, the orientation of magnetization in the reference layer of a prior magnetic memory cell is typically parallel to the easy axis of the data storage layer.
A magnetic memory cell is typically in a low resistance state if the orientation of magnetization in its data storage layer is parallel to the orientation of magnetization in its reference layer. In contrast, a magnetic memory cell is typically in a high resistance state if the orientation of magnetization in its data storage layer is anti-parallel to the orientation of magnetization in its reference layer.
A prior magnetic memory cell is usually written by applying external magnetic fields that rotate the orientation of magnetization in the data storage layer from one direction to the other along its easy axis. This causes the magnetic memory cell to switch between its high and low resistance states. The logic state of the magnetic memory cell may be determined during a read operation by measuring its resistance.
Unfortunately, the effects of the edge domains in such a magnetic memory cell usually cause the resulting orientation of magnetization in the data storage layer to move away from its easy axis. This usually reduces the difference between the high and low resistance state of the magnetic memory cell and decreases the signal obtained during read operations. Such a decrease in signal usually increases the bit error rate in an MRAM.
One prior solution for minimizing these negative effects of edge domains is to form the data storage layer as a rectangle with an elongated dimension along its easy axis. Such a structure usually increases easy axis contribution to the resulting orientation of magnetization in the data storage layer in comparison to contributions from the edge domains. Unfortunately, such a rectangular configuration usually requires more energy to flip the orientation of magnetization in the data storage layer during write operations, thereby causing an increase in power consumption in a MRAM that uses such a structure. In addition, such rectangular magnetic memory cells usually limit the overall memory cell density that may be obtained in an MRAM.